Add licensing information about protobuf-files

[lemenkov]

Copied verbatim from upstream.

[tomas-abrahamsson]

Thanks, makes sense. I'll include it. Only thing that I find is of concern is the end of the last paragraph, "This code is not
standalone and requires a support library to be linked with it. This support library is itself covered by the above license.", which is referring to the Google's protobuf lib, not this gpb where it would now be appearing. I'm thinking to prepend a sentence or two to the entire license text to make clear what this license is covering.

[tomas-abrahamsson]

I just realized that I forgot to include this PR in the release that I just made (4.21.2). Sorry about that and about the rather long delay. I pushed to your branch an added commit to address the concerns in my previous comment. Does it look ok to you? If so, then I'll merge it and make another release.

[lemenkov]

LGTM, please merge.

[tomas-abrahamsson]

Merged and included in 4.21.3. Thanks for the PR.